The present invention relates to an automatic equalizer for a digital transmission line using a transversal equalizer, in particular, relates to an automatic equalizer wherein the desired equalization characteristic is obtained utilizing the amplitude level of the cosine component based upon the measurement result on the frequency axis of the transmission line. According to the present invention, an automatic equalizer which provides the desired amplitude equalization and/or delay equalization is obtained utilizing the result of discrete measurement of the amplitude characteristic obtained in dB(decibel) on the frequency axis and a delay characteristic obtained in msec(millisecond) or .mu.sec(micro second).
In a prior art device, the characteristics of a transmission line are obtained first, and then the inverted characteristics are obtained as the response of the Discrete Fourier Transform on the time axis, and the result is utilized as a tap weight of a transversal equalizer or filter. However, although the result of measurements concerning the amplitude characteristic obtained from a prior measuring apparatus is, in general, based on common logarithms a prior equalizer requires measurement results based on a linear scale. Further, in a prior art device, the delay characteristic obtained from a measuring apparatus must be converted to a phase characteristic. Further, according to a prior equalizer, Discrete Fourier Transform is not actually applied to an automatic equalizer since a residual ripple characteristic, the so-called Gibbs phenomenon, remains in the equalizer characteristic.
On the other hand, the Japanese patent application No. 51-70335 (U.S. Pat. No. 4,125,899) proposed a solution to the above shortcoming concerning the equalization of delay characteristic. However, said newly developed art still has problems with respect to effective use of tap length of a transversal filter depending upon a transmission line to be equalized, since said art utilizes an arrangement of multiple transversal filters connected serially to one another.